Compound semiconductor light-emitting device and production method thereof

ABSTRACT

A pn-junction compound semiconductor light-emitting device is provided, which comprises a stacked structure including a light-emitting layer composed of an n-type or a p-type aluminum gallium indium phosphide and a light-permeable substrate for supporting the stacked structure, and the stacked structure and the light-permeable substrate being joined together, wherein the stacked structure includes an n-type or a p-type conductor layer, the conductor layer and the substrate are joined together, and the conductor layer is composed of a Group III-V compound semiconductor containing boron.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional of application Ser. No. 10/594,065 filed Sep. 26,2006, which is a 371 of PCT Application No. PCT/JP2005/006520 filed Mar.28, 2005, which claims priority from Japanese Application No.2004-095145, filed Mar. 29, 2004 and which claims benefit pursuant to 35U.S.C. §119(e)(1) of U.S. Provisional Application No. 60/559,429 filedon Apr. 6, 2004. The above-noted applications are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a pn-junction compound semiconductorlight-emitting device having a stacked structure including alight-emitting layer composed of an aluminum gallium indium phosphidemixed crystal (AlGaInP) and, more particularly to a pn-junction compoundsemiconductor light-emitting device attaining high emission intensity.

BACKGROUND ART

Light-emitting diodes (hereinafter also referred to as LEDs) having alight-emitting layer which is composed of an aluminum gallium indiumphosphide mixed crystal (compositional formula:(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, 0≧X≧1) and which is vapor-grown on ann-type or p-type gallium arsenide (GaAs) single-crystal substrate areknown to emit light having a wavelength corresponding to green light tored light (see, for example, Non-Patent Document 1).

Specifically, an LED having on a GaAs substrate a light-emitting layercomposed of an aluminum gallium indium phosphide mixed crystal((Al_(X)Ga_(1-X))_(0.5)In_(0.5)P: 0≦X≦1) (Y=0.5 in the abovecompositional formula) is employed.

In an LED having a light-emitting layer composed of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, in order to attain high emissionintensity, diffusion of device operation current over a wide area of thelight-emitting layer and effective extraction of light to the outsideare essential. Thus, a current diffusion layer and a window layer aregenerally provided on the light-emitting layer.

The window layer allows light emitted from the light-emitting layer totransmit to the outside. For example, an LED having a window layercomposed of gallium phosphide (GaP) is disclosed (see Patent Document1).

In an LED having a stacked structure vapor-grown on a GaAs substrate,light emitted from the light-emitting layer can be extracted only fromthe upper side of the LED, since the GaAs substrate is not transparentwith respect to emission wavelength. Thus, efficiency of extractinglight to the outside is not satisfactory, which is to be improved.

In order to solve the problem, a method for producing an LED has beenproposed. In the method, a substrate which is transparent with respectto emission wavelength is joined onto a stacked structure formed on aGaAs substrate, and the GaAs substrate provided for vapor-growth of thestacked structure is removed.

By virtue of the thus-joined substrate which is transparent with respectto emission wavelength, the LED produced through the above method allowslight emission from the upper side as well as from the backside and sideplanes, thereby attaining high light extraction efficiency.

There have been known such methods for producing an LED includingjoining, onto a stacked structure having a light-emitting layer, asemiconductor substrate which is transparent with respect to emissionwavelength (e.g., GaP, zinc selenide (ZnSe), or silicon carbide (SiC))(see, for example, Patent Documents 2 and 3).

Another disclosed technique for producing an LED includes joining, ontoa stacked structure, a GaP substrate which is transparent with respectto emission wavelength by the mediation of a transparent conductive filmsuch as indium tin complex oxide film (ITO) (see, for example, PatentDocument 4).

[Non-Patent Document]

Y. Hosokawa, Journal of Crystal Growth (Holland), 2000, Vol. 221, p.652-656

[Patent Document 1]

U.S. Pat. No. 5,008,718 specification

[Patent Document 2]

Japanese Patent No. 3230638

[Patent Document 3]

Japanese Patent Application Laid-Open (kokai) No. 2001-244499

[Patent Document 4]

Japanese Patent No. 2588849

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Patent Document 4 also discloses that, when a GaP substrate which istransparent with respect to emission wavelength is joined onto theuppermost surface of a stacked structure including a cladding layer anda current diffusion layer each composed of(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, joining must be performed by heating athigh temperature (830° C. or higher) (see Patent Document 4, paragraph[0007] in the specification).

Patent Document 2 also discloses that, when light-irradiation means suchas YAG laser for heating is not employed in combination, a semiconductorsubstrate which is transparent with respect to emission wavelength issuitably joined onto a stacked structure by heating at 300° C. to 900°C. (see Patent Document 2, paragraph [0035] in the specification).

Under such high-temperature conditions, a Group III-V compoundsemiconductor which is used for forming a stacked structure and whichcontains aluminum (Al)—susceptible to oxidation—(e.g., aluminum galliumindium phosphide mixed crystal ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P) oraluminum gallium arsenide (compositional formula: Al_(X)Ga_(Y)As, 0≦X,Y≦1, X+Y=1) is readily oxidized.

Therefore, a high-resistive layer composed of an oxide or anothersubstance is formed in a junction area between the stacked structure anda light-permeable substrate (e.g., GaP substrate) joined thereto. Such ahigh-resistance layer may disturb flow of device operation current.

In some stacked structures, a layer composed of an aluminum galliumindium phosphide mixed crystal ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P) isprovided other than the light-emitting layer. Generally, in order toimpart conductivity to the mixed crystal layer, an impurity elementwhich readily thermally diffuses such as zinc (Zn) or selenium (Se) isadded to the layer.

When a light-permeable substrate is joined to a stacked structurethrough heating at high temperature, an impurity element which readilythermally diffuses such as zinc (Zn) or selenium (Se) diffuses into alight-emitting layer or another layer. Thus, carrier concentration of ann-type or a p-type light-emitting layer, or forward voltage (Vf) of LEDsmay problematically vary.

Patent Document 4 discloses transparent conductive oxide films includingiodide tin oxide film and cadmium tin oxide film. However, these oxidefilms are difficult to attain reliable Ohmic contact with a Group III-Vcompound semiconductor such as an aluminum gallium indium phosphidemixed crystal ((Al_(X)Ga_(1-X))_(Y)In_(1-Y)P).

Thus, even when a transparent substrate having excellent lightpermeability such as sapphire (α-Al₂O₃ single-crystal), glass, titaniumdioxide (TiO₂), or magnesium oxide (MgO) is joined to a stackedstructure by the mediation of any of the aforementioned transparentoxide films, diffusion of device operation current by the mediation ofthe transparent substrate over a wide area of the stacked structure inthe produced LED is difficult, which is problematic.

DISCLOSURE OF THE INVENTION

The present invention has been conceived in order to solve theaforementioned problems involved in the conventional techniques. Thus,the invention provides a pn-junction compound semiconductorlight-emitting device which has low resistance, allows device operationcurrent to easily flow, and exhibits excellent efficiency of extractionof light to the outside, and a method for producing the device.

Accordingly, the present invention is directed to the following.

(1) A pn-junction compound semiconductor light-emitting devicecomprising a stacked structure including a light-emitting layer composedof an n-type or a p-type aluminum gallium indium phosphide and alight-permeable substrate for supporting the stacked structure, thestacked structure and the light-permeable substrate being joinedtogether, characterized in that the stacked structure includes an n-typeor a p-type conductor layer, that the conductor layer and the substrateare joined together, and that the conductor layer is composed of a GroupIII-V compound semiconductor containing boron.

(2) A pn-junction compound semiconductor light-emitting device asdescribed in (1) above, wherein the conductor layer has a bandgap atroom temperature which is greater than that of the light-emitting layer.

(3) A pn-junction compound semiconductor light-emitting device asdescribed in (1) or (2) above, wherein the conductor layer is composedof an undoped Group III-V compound semiconductor containing boron towhich an impurity element has not been intentionally added.

(4) A pn-junction compound semiconductor light-emitting device asdescribed in any one of (1) to (3) above, wherein the conductor layer iscomposed of a Group III-V compound semiconductor containing arsenic andboron.

(5) A pn-junction compound semiconductor light-emitting device asdescribed in any one of (1) to (4) above, wherein the conductor layer iscomposed of a Group III-V compound semiconductor containing phosphorusand boron.

(6) A pn-junction compound semiconductor light-emitting device asdescribed in (5) above, wherein the conductor layer is composed of boronphosphide.

(7) A pn-junction compound semiconductor light-emitting device asdescribed in any one of (1) to (6) above, wherein the conductor layer iscomposed of a boron-containing Group III-V compound semiconductorcontaining twins.

(8) A pn-junction compound semiconductor light-emitting device asdescribed in (7) above, wherein each of the twins has, as a twinningplane, a (111) lattice plane of a boron-containing Group III-V compoundsemiconductor.

(9) A method for producing a pn-junction compound semiconductorlight-emitting device characterized by comprising a step of forming astacked structure through sequentially stacking on a crystal substrate alower cladding layer, a light-emitting layer composed of n-type orp-type aluminum gallium indium phosphide, an upper cladding layer, andan n-type or a p-type conductor layer composed of a boron-containingGroup III-V compound semiconductor, and a step of joining the conductorlayer to a light-permeable substrate.

(10) A method for producing a pn-junction compound semiconductorlight-emitting device as described in (9) above, wherein the crystalsubstrate is removed after joining of the conductor layer to thelight-permeable substrate.

(11) A method for producing a pn-junction compound semiconductorlight-emitting device as described in (9) or (10) above, wherein theconductor layer is formed through crystal growth at a growth rate of 20nm/min to 30 nm/min until the conductor layer thickness reaches 10 nm to25 nm, followed by crystal growth at a growth rate less than 20 nm/minuntil the conductor layer comes to have a thickness of interest.

According to the pn-junction compound semiconductor light-emittingdevice of the present invention, the conductor layer is composed of aboron-containing Group III-V compound semiconductor. Thus, in thepn-junction compound semiconductor light-emitting device, the conductorlayer and the light-permeable substrate are joined to each other withhigh adhesion. On the conductor layer, an Ohmic electrode can bereliably formed.

Therefore, the invention provides a pn-junction compound semiconductorlight-emitting device which has low resistance, allows device operationcurrent to easily flow, and exhibits excellent efficiency of extractionof light to the outside.

Since the conductor layer has a bandgap at room temperature which isgreater than that of the light-emitting layer, the light emitted fromthe light-emitting layer can be caused to transmit to thelight-permeable substrate with low transmission loss, whereby highemission intensity can be attained.

Since the conductor layer is composed of an undoped Group III-V compoundsemiconductor containing boron to which an impurity element has not beenintentionally added, the phenomenon that the added impurity elementdiffuses into the light-emitting layer or other layers, thereby varyingforward voltage or other properties of a pn-junction compoundsemiconductor light-emitting device does not occur, and low forwardcurrent can be attained.

Since the conductor layer is composed of a Group III-V compoundsemiconductor containing arsenic and boron, an electrode exhibitingexcellent Ohmic contact characteristics can be formed on the conductorlayer, whereby low forward current can be attained.

Since the conductor layer is composed of a Group III-V compoundsemiconductor containing phosphorus and boron or a Group III-V compoundsemiconductor containing arsenic and boron (boron arsenide phosphide), awide bandgap is obtained and the light emitted from the light-emittinglayer can be caused to transmit to the light-permeable substrate withlower transmission loss, whereby higher emission intensity can beattained.

Since the conductor layer is composed of a boron-containing Group III-Vcompound semiconductor containing twins, lattice mismatch between theconductor layer and a base layer is mitigated, thereby yielding aconductor layer with high crystallinity. Thus, a pn-junction compoundsemiconductor light-emitting device which has lower resistance andexhibits excellent efficiency of extraction of light to the outside canbe produced.

According to the method for producing a pn-junction compoundsemiconductor light-emitting device, an n-type or a p-type conductorlayer composed of a boron-containing Group III-V compound semiconductoris formed to provide a joining layer for joining the stacked structureand the light-permeable substrate. Thus, the conductor layer and thelight-permeable substrate can be joined to each other at low temperaturewith strong adhesion without using a light-irradiation means such as YAGlaser for heating in combination.

According to the method, formation of a high-resistance layer composedof an oxide or another substance and thermal diffusion of an impurityelement added to a component layer of the stacked structure can beprevented, such phenomena conventionally occurring in the case where aconductor layer composed of gallium phosphide or a similar substance anda light-permeable substrate are joined at high temperature. On theconductor layer, an Ohmic electrode can be reliably formed.

According to the method, a pn-junction compound semiconductorlight-emitting device which has low resistance, allows device operationcurrent to easily flow, and exhibits excellent efficiency of extractionof light to the outside can be produced.

Since the crystal substrate is removed after joining of the conductorlayer to the light-permeable substrate, light absorption by the crystalsubstrate can be avoided, and a pn-junction compound semiconductorlight-emitting device which exhibits excellent efficiency of extractionof light to the outside can be produced.

According to the method, the conductor layer is formed through crystalgrowth at a growth rate of 20 nm/min to 30 nm/min until the conductorlayer thickness reaches 10 nm to 25 nm, followed by crystal growth at agrowth rate less than 20 nm/min until the conductor layer comes to havea thickness of interest. Therefore, twins are incorporated into theconductor layer, whereby a conductor layer with high crystallinity canbe formed.

Since crystal growth is continued at a growth rate less than 20 nm/minuntil the conductor layer comes to have a thickness of interest, aconductor layer having high surface flatness can be formed, whereby theconductor layer and the light-permeable substrate can be joined to eachother with high adhesion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic cross-sectional view of an exemplary stackedstructure of Example 1.

FIG. 2 shows a schematic cross-sectional view of an exemplary structureof a pn-junction compound semiconductor light-emitting device of Example1.

FIG. 3 shows a schematic plan view of an exemplary structure of thepn-junction compound semiconductor light-emitting device of Example 1.

FIG. 4 shows a schematic cross-sectional view of an exemplarylight-emitting device including the LED chip of Example 1.

FIG. 5 shows a schematic cross-sectional view of an exemplary lampincluding the LED chip of Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

A pn-Junction compound semiconductor light-emitting device will bedescribed.

The pn-junction compound semiconductor light-emitting device of thepresent invention comprises a stacked structure including alight-emitting layer composed of an n-type or a p-type aluminum galliumindium phosphide (compositional formula:(Al_(X)Ga_(1-X))_(0.5)In_(0.5)P, 0≦X≦1) and a light-permeable substratefor supporting the stacked structure. The stacked structure includes ann-conduction-type or a p-conduction-type conductor layer composed of aboron-containing Group III-V compound semiconductor and serving as ajoining layer with respect to the light-permeable substrate. Thelight-permeable substrate is joined to the conductor layer.

The stacked structure has a pn-junction double-hetero (DH) junctionstructure. An exemplary stacked structure includes a lower claddinglayer (e.g., p-type zinc (Zn)-doped (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P),a light-emitting layer (e.g., p-type undoped(Al_(0.4)Ga_(0.6))_(0.5)In_(0.5)P), and an upper cladding layer (e.g.,n-type selenium (Se)-doped (Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P), which aresequentially stacked.

On the conductor layer of the stacked structure, an Ohmic electrode of afirst polarity is provided, whereas an Ohmic electrode of the oppositepolarity is provided on another component layer (e.g., a buffer layer ora cladding layer) on the side opposite the conductor layer with respectto the light-emitting layer.

Through the above configuration, the light-emitting layer emits lightupon passage of forward device operation current between the Ohmicelectrodes.

When the stacked structure includes a lower cladding layer composed ofn-type (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, a light-emitting layer, an uppercladding layer composed of p-type (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, and aconductor layer composed of p-type boron phosphide, which aresequentially stacked, a p-type Ohmic electrode (positive electrode) isprovided on the conductor layer, and an n-type Ohmic electrode (negativeelectrode) is provided on the lower cladding layer, whereby apn-junction compound semiconductor light-emitting device is fabricated.

The conductor layer and the light-permeable substrate, which form thegist of the present invention, will next be described in detail.

The conductor layer is composed of a boron-containing Group III-Vcompound semiconductor.

As used herein, the term “boron-containing Group III-V compoundsemiconductor” refers to a Group III-V compound semiconductor containingas a component element boron (B). Examples include compounds representedby a compositional formula: B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ)(0<α1, 0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1); and compounds represented by acompositional formula: B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1,0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1).

When the boron-containing Group III-V compound semiconductor mixedcrystal contains a large variety of elements, a mixed crystal layerhaving consistent compositional proportions is more difficult to form(see Iwao Teramoto, “Introduction of Semiconductor Device,” Mar. 30(1995) Baihukan, 1st Ed. p. 24). Thus, the boron-containing Group III-Vcompound semiconductor represented by the above formula preferablycontains 3 or less component elements so as to form a desirable mixedcrystal layer having consistent compositional proportions.

Particularly preferably, the conductor layer is a Group III-V compoundsemiconductor layer which contains no component element that issusceptible to oxidation (e.g., aluminum (Al)) and which contains boronand phosphorus (P) or arsenic (As) as component elements. Since theconductor layer containing no component element that is susceptible tooxidation (e.g., aluminum (Al)) is highly resistive to oxidation, ahigh-resistance layer composed of an oxide or another substance whichwould otherwise be formed through heating of the conductor layer duringfabrication of a light-emitting device is prevented. Thus, lowering ofconductivity caused by formation of the high-resistance layer can beprevented.

Examples of the Group III-V compound semiconductor containing nocomponent element that is susceptible to oxidation (e.g., aluminum (Al))and containing boron and phosphorus as component elements (hereinafterthe semiconductor is also referred to as a boron-phosphide-basedsemiconductor) include boron monophosphide (BP), boron gallium phosphiderepresented by a compositional formula: B_(α)Ga_(γ)P (0<α≦1, 0≦γ<1),boron indium phosphide represented by a compositional formula:B_(α)In_(1-α)P (0<α≦1), and boron nitride phosphide represented by acompositional formula: BP_(1-δ)N_(δ) (0≦δ<1), which is a mixed crystalcontaining a plurality of Group V elements.

Since a boron-phosphide-based semiconductor containing phosphorusexhibits excellent heat resistance, a conductor layer formed therefromexhibits enhanced oxidation resistance.

Examples of the Group III-V compound semiconductor containing nocomponent element that is susceptible to oxidation (e.g., aluminum (Al))and containing boron and arsenic as component elements (hereinafter thesemiconductor is also referred to as a boron-arsenide-basedsemiconductor) include boron arsenide phosphide represented by acompositional formula: BP_(1-δ)As_(δ) (0≦δ<1).

A conductor layer composed of such a boron-arsenide-based semiconductorexhibits lower resistance as compared with a conductor layer composed ofa Group III-V compound semiconductor containing boron and phosphorus (P)serving as a sole Group V element. Through employment of aboron-arsenide-based semiconductor, forward voltage can be lowered.

No particular limitation is imposed on the boron atom concentration(content) of the conductor layer, and the concentration is appropriatelymodified in accordance with use, emission wavelength, or other factorsof the pn-junction compound semiconductor light-emitting device. Theconductor layer may be a layer which does not contain a large amount ofboron as a component element (e.g., a boron-doped Group III-V compoundsemiconductor).

When the boron atom concentration is less than 1×10¹⁹ cm⁻³, a conductorlayer exhibiting sufficient oxidation resistance is difficult toreliably form. Thus, in the below-mentioned step of producingpn-junction compound semiconductor light-emitting device, joining of aconductor layer and a light-permeable substrate is preferably performedin an oxygen-free inert gas atmosphere such as hydrogen (H₂), nitrogen(N₂), or argon (Ar).

The conduction type of the conductor layer is preferably caused tocoincide with that of a component layer of the stacked structure whichis in contact with the conductor layer (i.e., a base layer on which theconductor layer is formed).

The conductor layer preferably has a low resistance. Specifically, theconductor layer preferably has a carrier concentration at roomtemperature of 1×10¹⁹ cm³ or more and a resistivity at room temperatureof 5×10⁻² Ω·cm or less. The thickness of the conductor layer ispreferably adjusted to 50 nm to 5,000 nm.

Such a low-resistance conductor layer having such a thickness may beprovided in advance in the stacked structure so as to serve as a windowlayer through which the light emitted from the light-emitting layer istransmitted to the outside, as a current diffusion layer, or as asimilar layer.

The conductor layer preferably has a bandgap at room temperature whichis wider than that of the light-emitting layer. By virtue of the bandgapcharacteristics, the conductor layer absorbs substantially no lightemitted from the light-emitting layer and transmits the light to atransparent substrate, whereby excellent light extraction efficiency canbe attained. Thus, a light-emitting device which emits high-intensitylight can be fabricated.

The bandgap of the boron-containing Group III-V compound semiconductor(conductor layer) may be determined on the basis of photon energy (=h·ν)dependency of absorbance or on the basis of photon energy dependency ofa product (=2·n·k) of refractive index (n) and extinction coefficient(k).

When the conductor layer is composed of a boron-phosphide-basedsemiconductor or boron arsenide phosphide (one of boron-arsenide-basedsemiconductor), a wide bandgap can be attained.

Particularly preferably, the conductor layer is composed of boronmonophosphide, which attains a wide bandgap at room temperature of 2.8eV to 5.0 eV For example, a boron monophosphide conductor layer having abandgap at room temperature of 2.8 eV or more can be formed throughMOCVD at a formation rate of 2 nm/min to 30 nm/min.

When the conductor layer has a bandgap at room temperature in excess of5.0 eV, the energy gap between the conductor layer and thelight-emitting layer or the cladding layer excessively increases, whichis not preferred for lowering forward voltage or threshold voltage ofthe pn-junction compound semiconductor light-emitting device.

For example, when the light-emitting layer of a red-light-emittingpn-junction compound semiconductor light-emitting device is composed ofaluminum gallium indium phosphide which is represented by acompositional formula: (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P and has a bandgapat room temperature of 2.0 eV, a conductor layer composed of boronarsenide phosphide (BP_(1-δ)As_(δ): 0≦δ<1) and having a bandgap at roomtemperature of 2.3 eV can be employed.

When the conductor layer is composed of a boron-phosphide-basedsemiconductor or a boron-arsenide-based semiconductor, the semiconductorforming the conductor layer is preferably a semiconductor to which noimpurity element has intentionally been added (i.e., an undopedsemiconductor).

As compared with conventional semiconductor materials such as aluminumarsenide represented by a compositional formula: Al_(X)Ga_(Y)As (0≦X,Y≦1, X+Y=1) and aluminum gallium indium phosphide represented by acompositional formula: Al_(X)Ga_(Y)In_(z)P (0≦X, Y, Z≦1, X+Y+Z=1), aboron-phosphide-based semiconductor or a boron-arsenide-basedsemiconductor exhibits smaller ionic bond property. Therefore, eventhough the semiconductor is undoped, low resistance and wide bandgap canbe attained.

For example, when boron monophosphide (BP)—type of boron-phosphide-basedsemiconductor—is employed, a conductor layer having a carrierconcentration as high as 10¹⁹ cm⁻³ to 10²⁰ cm³ can be readily formed inan undoped state.

Conventionally, in some cases, a doped conductor layer to which animpurity element has been intentionally added (e.g., zinc (Zn)-dopedGaP) is provided. In a light-emitting device having such a conductivelayer, the impurity element (zinc) diffusing from the conductor layermay vary carrier concentration and conduction type of the light-emittinglayer. In this case, forward voltage (Vf) deviating from the voltage ofinterest may be applied, or light of a wavelength deviating from thewavelength of interest may be emitted.

In contrast, when a conductor layer composed of an undopedboron-phosphide-based semiconductor or boron-arsenide-basedsemiconductor is provided, the amount of impurity element diffusing fromthe conductor layer into a component layer of the stacked structurewhich is in contact with the conductor layer or into the light-emittinglayer can be reduced, whereby deterioration in characteristics of thelight-emitting layer which would otherwise be caused by diffusion ofexternal impurity elements can be prevented. In addition, diffusion ofdevice operation current over the light-emitting layer can befacilitated by virtue of low resistance.

Therefore, such a boron-containing Group III-V compound semiconductorlayer, having low resistance and wide bandgap even in an undoped state,may be suitably employed as a cladding layer which does not deterioratecharacteristics of an (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P light-emitting layerwhich would otherwise be caused by diffusion of external impurityelements or a current diffusion layer for diffusing device operationcurrent over the light-emitting layer. Furthermore, the boron-containingGroup III-V compound semiconductor layer having a wide bandgap may alsoserve as a light-transmitting layer through which light emitted from thelight-emitting layer is transmitted through the substrate to theoutside.

For this reason, the stacked structure may include the aforementionedboron-containing Group III-V compound semiconductor layer, having lowresistance and wide bandgap even in an undoped state, serving as acladding layer or a current diffusion layer, or the boron-containingGroup III-V compound semiconductor layer having a wide bandgap servingas a light-transmitting layer.

The conductor layer preferably contains twins.

When twins are formed in an area in the vicinity of the junctioninterface between a component layer of the stacked structure which is incontact with the conductor layer (i.e., a base layer on which theconductor layer is formed) and the conductor layer, lattice mismatchbetween the conductor layer and the base layer is mitigated, whereby aconductor layer composed of a boron-containing Group III-V compoundsemiconductor with few misfit dislocations can be formed.

Particularly, each of the twins more preferably has, as a twinningplane, a (111) crystal plane of a Group III-V compound semiconductorcontaining boron. In this case, lattice mismatch between the conductorlayer and the base layer is further mitigated.

Next, the light-permeable substrate for supporting the stacked structurewill be described in detail.

The light-permeable substrate is composed of a material which istransparent with respect to emission wavelength. The light-permeablesubstrate is preferably formed of a glass material, whichever conductiontype and material of the conductor layer are selected.

Examples of the glass material include silica glass (see Shiro Yoshizawaet al., Industrial chemistry basic lecture 5 “Inorganic IndustrialChemistry,” Asakura-shoten, Feb. 25 (1973), 6th ed., p. 169); silicateglass such as soda-lime glass (see the above “Inorganic IndustrialChemistry,” p. 205-206); borosilicate glass in which silica is partiallysubstituted by boron oxide (see the above “Inorganic IndustrialChemistry,” p. 207), and other amorphous glass materials. Specificexamples include 96% silica glass.

Particularly, the light-permeable substrate is preferably composed of aglass material having small thermal expansion coefficient such asborosilicate glass (see the above “Inorganic Industrial Chemistry,” p.208) or glass-ceramics. By use of such a substrate, thermal stressbetween the light-permeable substrate and the stacked structure joinedto the light-permeable substrate can be mitigated. Thus, even when alight-emitting device includes a light-emitting layer composed of, forexample (Al_(X)Ga_(1-X))_(0.5)In_(0.5)P, cracking of the stacked layerwhich would otherwise be caused by thermal stress can be prevented,thereby attaining excellent thermal stability.

The light-permeable substrate preferably has a refractive index which issmaller than that of the boron-containing Group III-V compoundsemiconductor. Specifically, the light-permeable substrate preferablyhas a refractive index of 1.3 or more and less than 2.0, more preferably1.5 to 1.8.

Examples of optical glass material forming the light-permeable substratehaving a refractive index of 1.5 to 1.8 with respect to sodium (Na)d-ray (587 nm), which can be emitted from a light-emitting layerrepresented by a compositional formula: (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P,include crown (K), borosilicate crown (BK), barium crown (BaK), flint(F), barium flint (BaF), lanthanum crown (LaK), lanthanum flint (LaF)(see the above “Inorganic Industrial Chemistry,” p. 214).

Other than glass materials, the light-permeable substrate may be formedof a material which is permeable with respect to light emitted from alight-emitting layer represented by a compositional formula:(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P and which allows the light emitted fromthe light-emitting layer to transmit without absorption.

Examples of the non-glass material forming the light-permeable substrateinclude Group II-VI compound semiconductors such as zinc oxide (ZnO),zinc sulfide (ZnS), and zinc selenide (ZnSe); silicon carbide (SiC) ofcubic 3C-type, hexagonal 4H-type, hexagonal 6H-type, or 15R-type;sapphire (α-Al₂O₃ single crystal); gallium nitride (GaN); and aluminumnitride (AlN).

When the light-permeable substrate contains a conductive substance suchas GaN or ZnSe, the conduction type of the light-permeable substratepreferably coincides with that of the conductor layer.

[Method for Producing a Pn-Junction Compound SemiconductorLight-Emitting Device]

Firstly, a stacked structure is formed through sequentially stacking ona crystal substrate a lower cladding layer, a light-emitting layercomposed of n-type or p-type aluminum gallium indium phosphide, an uppercladding layer, an n-type or a p-type conductor layer composed of aboron-containing Group III-V compound semiconductor.

Examples of the crystal substrate include a silicon (Si) crystal,sapphire (α-Al₂O₃ single-crystal), hexagonal or cubic silicon carbide(SiC), gallium nitride (GaN), gallium arsenide (GaAs), and those crystalsubstrates having thereon a base layer composed of a Group III-Vsemiconductor layer.

The lower cladding layer, light-emitting layer, and upper cladding layermay be formed through a conventional vapor phase growth means such asMOCVD (metal-organic chemical vapor deposition). Needless to say, abuffer layer composed of a Group III-V semiconductor such as galliumarsenide (GaAs) may be formed on the crystal substrate, followed byforming these component layers.

The conductor layer composed of a boron-containing Group III-V compoundsemiconductor is formed on the upper cladding layer through a vaporphase growth means such as the halogen method, the hydride method, orMOCVD (metal-organic chemical vapor deposition), or molecular-beamepitaxy (see J. Solid State Chem., 133 (1997), p. 269-272).

For example, a conductor layer composed of p-type or n-type boronmonophosphide (BP) may be formed through an atmospheric pressure (nearatmospheric pressure) or reduced-pressure MOCVD by use of triethylborane(molecular formula: (C₂H₅)₃B) and phosphine (molecular formula: PH₃) assources.

During formation of the conductor layer composed of p-type boronmonophosphide (BP), the formation temperature is preferably 1,000° C. to1,200° C., and the source supply ratio (V/III ratio; e.g., PH₃/(C₂H₅)₃B)is preferably 10 to 50.

During formation of the conductor layer composed of n-type boronmonophosphide (BP), the formation temperature is preferably 700° C. to1,000° C., and the V/III ratio is preferably 200 or more, morepreferably 400 or more.

Through precise control of formation rate in addition to formationtemperature and V/III ratio, there can be formed a conductor layercomposed of a boron-phosphide-based semiconductor which exhibits a widebandgap at room temperature.

Particularly when the formation rate during MOCVD is controlled to 2nm/min to 30 nm/min, a conductor layer which is composed of boronmonophosphide and which exhibits a bandgap at room temperature of 2.8 eVor more can be produced.

As described below, a conductor layer of high crystallinity can beformed through employment of an increased growth rate in an initialstage of formation of the conductor layer.

One exemplary boron-containing Group III-V compound semiconductor forforming the conductor layer is boron phosphide. Thesphalerite-crystal-type boron phosphide has a lattice constant of 0.454nm, and sphalerite-crystal-type boron arsenide (BAs) has a latticeconstant of 0.477 nm. Thus, these lattice constants do not match thelattice constant of (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P serving as alight-emitting layer or cladding layer.

For example, gallium phosphide (GaP) has a lattice constant of 0.545 nm,and lattice mismatch between (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P and boronphosphide is about 16.7%, based on gallium phosphide (GaP).

In the case where a conductor layer composed of a boron-containing GroupIII-V compound semiconductor (e.g., boron phosphide) is deposited on acladding layer having such a high mismatch degree, a desirable conductorlayer of high crystallinity can be produced through increasing thegrowth rate in an initial stage of the growth.

For example, when a conductor layer composed of undoped boron phosphideis formed on a layer such as a cladding layer composed of undoped(Al_(0.5)Ga_(0.5))_(0.5)In_(0.5)P at 700° C. to 950° C., the growth ratein an initial stage of the growth is preferably 20 nm/min to 30 nm/min.

As used herein, the growth rate is a value derived by dividing the layerthickness of the grown conductor layer by the time required forobtaining the thickness.

The aforementioned increased growth rate is exclusively employed untilthe layer thickness reaches 10 nm to 25 nm. Subsequently, crystal growthis continued at a reduced growth rate of less than 20 nm/min until athickness of interest is obtained, thereby forming a conductor layer.

When a conductor layer is grown at a growth rate in excess of 30 nm/minuntil a thickness of interest is obtained, the formed conductor layerhas a disordered surface with less flatness. Such a surface is notpreferred, since sufficient adhesion to the below-mentionedlight-permeable substrate fails to be attained.

When a boron-containing Group III-V compound semiconductor forming theconductor layer is grown on, for example, a cladding layer composed of,for example, Al_(X)Ga_(Y)As (0≦X, Y≦1, X+Y=1) at the aforementioned highgrowth rate, twins can be formed in a joining layer (in an area in thevicinity of the junction interface between the cladding layer (or asimilar layer) and the conductor layer).

The twins formed in the conductor layer (in an area in the vicinity ofthe junction interface between the cladding layer (or a similar layer)and the conductor layer) can mitigate lattice mismatch between thecladding layer (or a similar layer) and the conductor layer, whereby aconductor layer containing few misfit dislocations can be formed.

Particularly, through formation of twins each having, as a twinningplane, a (111) lattice plane of a boron-containing Group III-V compoundsemiconductor, lattice mismatch is further mitigated.

Particularly when the aforementioned growth rate in an initial growthstage is controlled to 20 nm/min to 30 nm/min, twins each having a (111)lattice plane as a twinning plane can be generated.

As described above, a stacked structure having a conductor layer servingas a joining layer with respect to the light-permeable substrate can beformed.

Next, the conductor layer of the stacked structure is joined to thelight-permeable substrate through the following procedure.

Examples of the light-permeable substrate include Group II-VI compoundsemiconductors such as zinc oxide (ZnO), zinc sulfide (ZnS), and zincselenide (ZnSe); silicon carbide (SiC) of cubic 3C-type, hexagonal4H-type, hexagonal 6H-type, or 15R-type; sapphire (α-Al₂O₃single-crystal); gallium nitride (GaN); and aluminum nitride (AlN). Whenthe substrate composed of such a single crystal is employed, theconductor layer and the light-permeable substrate are joined to eachother such that the mismatch in crystal lattice spacing therebetween ispreferably reduced as small as possible. By virtue of the joiningfeature, stress applied to the light-emitting layer during joining ofthe joining layer and the light-permeable substrate can be reduced.

For example, the interplane spacing of a (110) lattice plane of boronmonophosphide (lattice constant=0.454 nm) is 0.320 nm, whilewurtzite-type crystal gallium nitride has an a-axis lattice constant of0.319 μm. Thus, when a conductor layer composed of boron monophosphideis joined to a light-permeable substrate composed of a gallium nitride(0001) crystal plane, the conductor layer and the substrate are heatedat, for example, 450° C., with positioning such that the (110) latticeplane of boron monophosphide forming the joining layer and the a-axis ofGaN forming the light-permeable substrate are arranged in the samedirection.

When a light-permeable substrate composed of a glass material is used,the conductor layer and the light-permeable substrate may be joined toeach other through an anodic joining means.

When the conductor layer and the light-permeable substrate are joined toeach other through an anodic joining means, the negative (−) voltageapplied to a glass plate serving as the light-permeable substrate ispreferably 100 V to 1,200 V. As the applied voltage increases, joiningcan be performed easier. However, yield of joined products decreases.Therefore, the applied voltage is preferably 200 V to 700 V, morepreferably 300 V to 500 V.

Upon joining through an anodic joining means, the conductor layer andthe light-permeable substrate is preferably joined to each other whileheating the layer and the substrate. Heating facilitates joining.

The heating temperature is preferably 200° C. to 700° C. When theheating temperature during joining is higher, the voltage applied to theconductor layer and the light-permeable substrate is required to belowered.

In the case where the conductor layer and the light-permeable substrateare joined to each other through an anodic joining means, thelight-permeable substrate is preferably composed of a glass materialcontaining an alkaline component. Examples of such a glass materialinclude borosilicate glass such as soda-lime glass.

Among them, borosilicate glass, which contains boron as a component,provides excellent adhesion with respect to a Group III-V compoundsemiconductor layer also containing boron as a component element. Thesubstrate composed of a glass material preferably has a thickness of 0.1mm to 1.0 mm.

Alternatively, the conductor layer and the light-permeable substrate maybe joined to each other by use of a conductive liquid (paste or gel)containing a conductive oxide powder.

In one specific mode, the conductor layer and the light-permeablesubstrate are joined to each other by use of a conductive gel containingindium tin complex oxide through a sol-gel means.

When a compound semiconductor device has a conductor layer having a widebandgap allowing the light emitted from the light-emitting layer tosufficiently transmit, a metallic material coating film that reflectsthe light emitted from the light-emitting layer may be formed on ajoining surface of the conductor layer or the light-permeable substrate,and the conductor layer and the light-permeable substrate may be joinedby use of a conductive paste.

For example, a coating film of a metallic material such as any of thesix metals belonging to the platinum group including platinum (Pt),iridium (Ir), and rhodium (Rh) (see “Duffy Inorganic Chemistry,”Hirokawa-shoten, Apr. 15 (1971), 5th ed., p. 249), silver (Ag), chromium(Cr), etc. is formed on a conductor layer. The metallic-film-coatedsurface of the conductor layer is disposed so as to oppose alight-permeable substrate (e.g., glass substrate) and joined to thelight-permeable substrate with a conductive paste. Through formation ofa light-reflective metallic film on a joining surface of the conductorlayer or the light-permeable substrate, a flip-mounted pn-junctioncompound semiconductor light-emitting device for emitting high-intensitylight can be fabricated.

On the conductor layer, an Ohmic electrode of a first polarity isformed, whereas an Ohmic electrode of the opposite polarity is providedon another component layer of the stacked structure (e.g., a bufferlayer or a cladding layer) on the side opposite the conductor layer withrespect to the light-emitting layer. The Ohmic electrodes may be formedthrough any known method such as sputtering or vapor deposition.

For example, when the stacked structure includes a lower cladding layercomposed of n-type (Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, a light-emittinglayer, an upper cladding layer composed of p-type(Al_(X)Ga_(1-X))_(Y)In_(1-Y)P, and a conductor layer composed of p-typeboron phosphide, which are sequentially stacked, a p-type Ohmicelectrode (positive electrode) is provided on the conductor layer, andan n-type Ohmic electrode (negative electrode) is provided on anothercomponent layer on the side opposite the conductor layer with respect tothe light-emitting layer; i.e., on the lower cladding layer.

On the n-type conductor layer composed of, for example,boron-phosphide-based semiconductor or a boron-arsenide-basedsemiconductor, an n-type Ohmic electrode may be formed from a gold (Au)alloy such as gold (Au)-germanium (Ge).

On the p-type conductor layer composed of, for example,boron-phosphide-based semiconductor or a boron-arsenide-basedsemiconductor, a p-type Ohmic electrode may be formed fromconventionally employed nickel (Ni) (see DE (west Germany) U.S. Pat. No.1,162,486), nickel alloy, gold (Au)-zinc (Zn) alloy, gold (Au)-beryllium(Be) alloy, or the like.

When the Ohmic electrode having a multi-layer structure is formed, theuppermost layer is preferably formed of gold (Au) or aluminum (Al) inorder to facilitate bonding. In the case where the Ohmic electrodehaving a tri-layer structure is formed, an intermediate layer providedbetween the bottom portion and the uppermost layer is formed of atransition metal (e.g., titanium (Ti) or molybdenum (Mo)) or platinum(Pt).

As described above, through formation of Ohmic electrodes after joiningof the conductor layer and the light-permeable substrate, a pn-junctioncompound semiconductor light-emitting device is fabricated. In thepresent invention, after joining of the conductor layer and thelight-permeable substrate, the crystal substrate used for vapor-growingof the stacked structure is preferably removed. Through removal of thecrystal substrate, a pn-junction compound semiconductor light-emittingdevice exhibiting high efficiency of extraction of light to the outsidecan be produced.

Particularly when the crystal substrate is a GaAs substrate, which has anarrow bandgap and absorbs light emitted from the light-emitting layer,a pn-junction compound semiconductor light-emitting device exhibitinghigh emission intensity can be produced through removal of the crystalsubstrate.

The crystal substrate may be removed through a conventional etchingtechnique. Specifically, a GaAs crystal substrate can be removed throughwet-etching with a liquid mixture containing aqueous ammonia and aqueoushydrogen peroxide.

In contrast, when the crystal substrate is, for example, a galliumphosphide (GaP) substrate, which is composed of a material allowinglight emitted from the light-emitting layer to transmit, a pn-junctioncompound semiconductor light-emitting device exhibiting high emissionintensity can be produced without intentionally removing the crystalsubstrate.

For example, since a gallium phosphide crystal substrate hasconductivity, an Ohmic electrode of a first polarity is provided on thebackside of the gallium phosphide crystal substrate, and an Ohmicelectrode of the opposite polarity is provided on a stacked structurecomponent layer (e.g., a conductor layer), whereby a pn-junctioncompound semiconductor light-emitting device exhibiting high emissionintensity can be produced.

The method of the present invention for producing a pn-junction compoundsemiconductor light-emitting device will next be described in moredetail, with reference to an exemplary method for producing apn-junction compound semiconductor light-emitting device. In the method,a stacked structure is formed on a crystal substrate which absorbs lightemitted from an n-type light-emitting layer (e.g., GaAs substrate), anda pn-junction compound semiconductor light-emitting device exhibitinghigh efficiency of extraction of light to the outside is produced fromthe stacked structure.

(1) Through an MOCVD means, a lower cladding layer composed of p-type(Al_(X)Ga_(1-X))_(0.5)In_(0.5)P, a light-emitting layer composed of(Al_(X)Ga_(1-X))_(0.5)In_(0.5)P, and an upper cladding layer composed ofn-type (Al_(X)Ga_(1-X))_(0.5)In_(0.5)P are sequentially stacked on acrystal substrate, for example, a zinc-doped p-type GaAs crystalsubstrate, to thereby form a double-hetero (DH) junction light-emittingportion ((J. Korean Association of Crystal Growth), 2001, Vol. 11, No.5, p. 207-210).

Needless to say, p-type GaAs buffer layer may be formed on the p-typezinc-doped GaAs crystal substrate.

(2) Subsequently, on the upper cladding layer of the double-hetero (DH)junction light-emitting portion, a conductor layer composed of n-typeundoped boron phosphide is vapor-phase-grown through an MOCVD means, tothereby form a stacked structure including the double-hetero (DH)junction light-emitting portion and the conductor layer.

(3) Then, the conductor layer serving as the uppermost surface of thestacked structure and a colorless transparent substrate composed oflow-melting-point glass are joined to each other through an anodicjoining means.

(4) The GaAs substrate used for forming the stacked structure is removedfrom the stacked structure through etching.

Thereafter, Ohmic electrodes are formed through the following procedure,to thereby fabricate a light-emitting device.

(5) A p-type Ohmic electrode is formed directly on a surface of thep-type GaAs buffer layer or the lower cladding layer, the surface beingexposed through removal of the GaAs substrate.

(6) Thereafter, a portion throughout the lower cladding layer,light-emitting layer, and upper cladding layer corresponding to the areawhere an n-type Ohmic electrode is to be formed is removed throughetching, thereby exposing the aforementioned n-type boron phosphidelayer.

(7) on the thus-exposed conductor layer, an n-type Ohmic electrode isdirectly formed, to thereby fabricate a pn-junction compoundsemiconductor light-emitting device.

As described above, a pn-junction compound semiconductor light-emittingdevice which emits light through plane on the light-permeable substrateside can be produced by removing a GaAs substrate and forming one Ohmicelectrode on the conductor layer and the other Ohmic electrode onanother component layer on the side opposite the conductor layer withrespect to the light-emitting layer. Through employment of thepn-junction compound semiconductor light-emitting device, aflip-chip-type light-emitting device can be fabricated.

For example, a so-called flip-mounted-type light-emitting device,employing extraction of light emitted from a light-emitting layerthrough a light-permeable substrate, may be fabricated through thefollowing procedure. An n-type Ohmic electrode and a p-type Ohmicelectrode are provided such that the two electrodes are disposed to faceto the circuit substrate, and the light-permeable substrate faces upward(outwardly). A metal bump electrode is formed on each Ohmic electrode,and the n-type and the p-type Ohmic electrodes are connected to thecircuit substrate via the metal bumps.

Alternatively, a similar light-emitting device may also be fabricated bymounting a pn-junction compound semiconductor light-emitting device on astem, while a light-permeable substrate faces the stem, followed bybonding the n-type and the p-type Ohmic electrodes to the correspondingexternal electrodes, respectively. In this case, when a reflectingmirror is provided on the stem so as to reflect the light emitted fromthe light-emitting layer through the light-permeable substrate, thelight emitted from the light-emitting layer can be fully utilized, andhigh-luminance light-emitting devices such as an LED lamp and a lightsource can be fabricated.

EXAMPLES Example 1

The present invention will next described in detail by way of Example 1,in which a conductor layer composed of undoped n-type boron arsenidephosphide and a light-permeable substrate composed of a glass materialare joined to each other, to thereby form a pn-junction compoundsemiconductor light-emitting device.

FIG. 1 is a schematic cross-sectional view of an exemplary stackedstructure 11 having a pn-junction double-hetero (DH) junction structureand formed on a crystal substrate.

Firstly, a stacked structure 11 forming the pn-junction compoundsemiconductor light-emitting device 10 (hereinafter the device may bereferred to as an LED chip) was formed through the following procedure.

The stacked structure 11 was formed through sequentially stacking on a(100) crystal plane of a zinc (Zn)-doped p-type gallium arsenide (GaAs)single-crystal substrate 100 the following layers: a zinc-doped p-typeGaAs buffer layer 101, a lower cladding layer 102 composed of azinc-doped aluminum gallium indium phosphide mixed crystal((Al_(0.70)Ga_(0.30))_(0.50)In_(0.50)P), an undoped n-typelight-emitting layer 103 composed of(Al_(0.14)Ga_(0.86))_(0.50)In_(0.50)P, and a selenium (Se)-doped n-typeupper cladding layer 104 composed of(Al_(0.70)Ga_(0.30))_(0.50)In_(0.50)P (see J. Korean Association ofCrystal Growth, 11(5)(2001), p. 207-210).

The layers 101 to 104 were vapor-phase-grown on the substrate 100 at720° C. through a conventional reduced-pressure MOCVD means.

On the upper cladding layer 104, an undoped n-type boron arsenidephosphide (BAs_(0.08)P_(0.92)) layer was deposited, to thereby form aconductor layer 105.

The conductor layer 105 composed of undoped n-type boron arsenidephosphide (BAs_(0.08)P_(0.92)) was formed through an atmosphericpressure (near atmospheric pressure) metal-organic chemical vapordeposition (MOCVD) means by use of triethylborane (molecular formula:(C₂H₅)₃B) as a boron (B), arsine gas (molecular formula: AsH₃) as anarsenic (As) source, and phosphine (molecular formula: PH₃) as aphosphorus (P) source. The thickness of the conductor layer 105 wasadjusted to 850 nm.

The method for forming the conductor layer 105 will next be described indetail.

The same conditions under which boron monophosphide (BP) having abandgap at room temperature of about 4.3 eV was formed were employed.That is, crystal growth of undoped n-type boron arsenide phosphide(BAs_(0.08)P_(0.92)) was initiated under the conditions: a V/III ratio((AsH₃+PH₃)/(C₂H₅)₃B) concentration ratio) of 800, a growth temperatureof 700° C., and a growth rate of 25 nm/min.

Crystal growth of undoped n-type boron arsenide phosphide(BAs_(0.08)P_(0.92)) was carried out at a growth rate of 25 nm/min foreight minutes. When the layer thickness reached 200 nm, the growth rateis lowered to 15 nm/min, and crystal growth was continued at the loweredgrowth rate.

Finally, when the thickness of the conductor layer 105 reached 850 nm,crystal growth was terminated.

The thus-formed conductor layer 105 was found to have a bandgap at roomtemperature of 3.5 eV. The carrier concentration and the resistivity atroom temperature were found to be 1×10²⁰ cm⁻³ and 2×10⁻² cm,respectively.

The conductor layer 105 was found to have a flat surface. The flatsurface was considered to be formed through reducing of theinitial-stage growth rate during formation of the conductor layer 105.

A transmission electron diffraction (TED) pattern of an area in thevicinity of the junction interface between the conductor layer 105 andthe upper cladding 104 was captured. In the TED pattern, extradiffraction spots appeared in an ordered pattern along the axis on which(111) diffraction spots appeared. The anomalous diffraction spots wereattributed to twins each having a (111) crystal plane as a twinningplane.

Through a conventional cross-section TEM technique, the inside structureof the conductor layer 105 was observed. The results indicated that alarge amount of twins each having a (111) crystal plane serving as atwinning plane were present particularly in the junction interfacebetween the conductor layer 105 and the upper cladding 104.

The conductor layer 105 of the stacked structure 11 was joined to thelight-permeable substrate 106 through the following procedure, tothereby form a pn-junction compound semiconductor light-emitting device.

FIG. 2 is a schematic cross-sectional view of an exemplary structure ofa pn-junction compound semiconductor light-emitting device 10, and FIG.3 a schematic plan view of an exemplary structure of the pn-junctioncompound semiconductor light-emitting device 10.

The conductor layer 105 and a light-permeable substrate 106 composed ofa colorless transparent borosilicate glass plate were joined to eachother through an anodic joining means. Joining conditions during anodicjoining included an applied voltage of 800 V and a temperature of 500°C. The employed light-permeable substrate 106 had a thickness of 0.15mm, a thermal expansion coefficient of about 6×10⁻⁶/K, and a refractiveindex of 1.3.

As described above, the conductor layer 105 had a flat surface.Therefore, the conductor layer 105 and the light-permeable substrate 106could be joined to each other with high adhesion.

After joining of the conductor layer 105 and the light-permeablesubstrate 106, the GaAs crystal substrate 100 employed for forming thestacked structure 11 was removed through etching with an aqueousammonium-hydrogen peroxide (H₂O₂) mixture.

Subsequently, the GaAs buffer layer 101 was removed through etching, tothereby expose a surface of the lower cladding layer 102. On the entiresurface of the exposed lower cladding layer 102, gold (Au)-beryllium(Be) alloy film, nickel (Ni) film, and gold (Au) film were sequentiallydeposited through conventional vacuum evaporation or electron-beamdeposition.

Through selective patterning based on a known photolithographictechnique, as shown in FIG. 2, a p-type Ohmic electrode 107 also servingas a pad electrode for wiring was provided on a corner portion of thetop surface of the lower cladding layer 102.

A portion throughout the lower cladding layer 102, the light-emittinglayer 103, and the upper cladding layer 104 corresponding to the areawhere an n-type Ohmic electrode 108 was to be formed was removed throughetching, thereby exposing the surface (the surface opposite the junctioninterface with the light-permeable substrate 106) of the conductor layer105.

Through a known photolithographic technique and selective patterning,the n-type Ohmic electrode 108 composed of gold-germanium (Au.Ge)vacuum-evaporated film was formed on the surface of the conductor layer105 exposed through etching.

The stacked structure 11 was cut, to thereby produce pn-junctioncompound semiconductor light-emitting devices (LED chips) 10 each havinga square (300 μm×300 μm) shape in plan view.

FIG. 4 is a schematic cross-sectional view of an exemplarylight-emitting device including the LED chip of Example 1.

A support 109 on which wiring circuits 109 a and 109 b were patternedwas provided.

The LED chip 10 was temporally fixed such that the light-permeablesubstrate 106 faced upward and the p-type and n-type Ohmic electrodes107 and 108 faced opposite the wiring circuits 109 b and 109 a,respectively. While the position was maintained, the p-type and n-typeOhmic electrodes 107 and 108 were electrically connected to the wiringcircuits 109 b and 109 a, respectively, by the mediation of metal bumps110, whereby the LED chip 10 was mounted on the support 109.

Subsequently, the thus-mounted LED chip 10 was encapsulated with acolorless, transparent epoxy resin 111, thereby fabricating alight-emitting device 12.

Upon encapsulating the LED chip 10 with the epoxy resin 111, the epoxyresin 111 was shaped such that the upper surface and the side surfacesof the light-permeable substrate 106 serving as light-emitting surfacesof the LED chip 10 were surrounded by a hemi-spherical lens having asemi-circular cross-section and such that the vertex of the hemi-spherewas on the center axis of the LED chip 10.

When a forward device operation current (20 mA) was caused to flowbetween the p-type and the n-type Ohmic electrodes 107 and 108 throughwires 109 a and 109 b provided on the support 109, the LED chip 10emitted a yellowish green light having a center wavelength of about 610nm.

The conductor layer 105 was formed from boron arsenide phosphide havinga wide bandgap and low resistance, and the light-permeable substrate 106was provided in the LED chip 10. Therefore, light emission was visuallyobserved in virtually the entire surface of the light-emitting layer 103other than the projection area of the p-type Ohmic electrode 107.

A near field pattern of the emitted light indicated that the lightemitted from the light-emitting layer 103 other than the aboveprojection area had virtually uniform intensity.

The luminance (emission intensity) of the light emitted from each chip,as determined through a typical integrating sphere, was 320 mcd.Furthermore, by virtue of the n-type Ohmic electrode 108 provideddirectly on the low-resistance conductor layer 105, forward voltage (Vf)was found to be as low as 2.3 V, whereas a high reverse voltageexceeding 8 V was attained at a reverse current of 10 μA.

As described above, the LED chip 10 according to the present inventionexhibits low forward current and resistance, facilitates flow of deviceoperation current, and exhibits high efficiency of extracting light tothe outside. Thus, the LED chip can emit high-intensity light.

Through employment of such an LED chip, a light-emitting device whichcan emit high-intensity light can be provided.

Example 2

An LED chip 20 is different from the LED chip of Example 1 in that anundoped n-type boron phosphide layer is provided as a conductor layer205.

The present invention will next be described by way of Example 2. Thesame component members as employed in Example 1 are represented by thesame reference numerals.

FIG. 5 is a schematic cross-sectional view of an exemplary LED lamp 22including the LED chip 20 of Example 2.

In a manner similar to that of Example 1, component layers 101 to 104 ofa stacked structure 21 other than the conductor layer 205 were formed ona single-crystal substrate 100.

Subsequently, an undoped n-type boron phosphide (BP) layer serving asthe conductor layer 205 was formed on an upper cladding layer 104.

The conductor layer 205 composed of undoped n-type boron phosphide (BP)was formed at 800° C. through an atmospheric pressure (near atmosphericpressure) metal-organic chemical vapor deposition (MOCVD) means by useof triethylborane (molecular formula: (C₂H₅)₃B) as a boron (B) andphosphine (molecular formula: PH₃) as a phosphorus (P) source. Thethickness of the conductor layer 205 was adjusted to 750 nm.

The thus-formed conductor layer 205 was found to have a carrierconcentration and the resistivity of 8×10¹⁹ cm⁻³ and 6×10⁻² Ω·cm,respectively.

The refractive index and extinction coefficient of the conductor layer205 were determined by use of a conventional ellipsometer, and thebandgap at room temperature of the conductor layer 205, as calculatedfrom the determined refractive index and extinction coefficient, wasabout 4.8 eV. Thus, the bandgap assures transmission of the lightemitted from the light-emitting layer 103.

In a manner similar to that of Example 1, the conductor layer 205serving as the uppermost surface of the stacked structure 21 was joinedto the light-permeable substrate 106 composed of a borosilicate glassplate through an anodic joining means.

After joining of the substrate 106, a GaAs crystal substrate 100 wasremoved, to thereby expose a surface of the lower cladding layer 103.

On the thus-exposed surface of the lower cladding layer 103, a p-typeOhmic electrode 107 having an Au—Ge/Ni/Au tri-layer structure wasprovided at the same position as that of Example 1 as shown in FIG. 2.

A portion throughout the lower cladding layer 102, light-emitting layer103, and upper cladding layer 104 corresponding to the area where ann-type Ohmic electrode 108 was to be formed was removed through etching,thereby exposing the surface (the surface opposite the junctioninterface with the light-permeable substrate 106) of the conductor layer205.

Through a known photolithographic technique and selective patterning,the n-type Ohmic electrode 108 composed of gold-beryllium (Au.Be)vacuum-evaporated film was formed on the exposed surface of theconductor layer 105.

The stacked structure 21 was cut, to thereby produce LED chips 20 eachhaving a square (400 μm×400 μm) in plan view.

A support 109 having a surface coated with silver (Ag) film 112 wasprovided. As shown in FIG. 5, the LED chip 20 was mounted on the Ag film112 of the support 109 such that the light-permeable substrate 106served as a lower layer (i.e., in contact with the support 109).

Subsequently, the p-type and the n-type Ohmic electrodes 107 and 108were individually wired so as to electrically connected to wiringcircuits, respectively (not illustrated in FIG. 5). The thus-formed LEDchip 20 was encapsulated with epoxy resin, to thereby fabricate an LEDlamp 22.

When a forward device operation current (20 mA) was caused to flowbetween the p-type and the n-type Ohmic electrodes 107 and 108, theforward voltage was lowered to 2.3 eV, and a high reverse voltage of 8 Vwas attained at a reverse current of 10 μA, indicating excellentrectifying characteristics.

When a forward device operation current (20 mA) was caused to flow, theLED chip 20 emitted an reddish orange light having a center wavelengthof about 610 nm. The luminance (emission intensity) of the light emittedfrom the LED lamp 22, as determined through a typical integratingsphere, was about 340 mcd.

The above results indicated that, through employment of the LED chip 20according to the present invention, an LED lamp 22 which can emithigh-intensity light can be provided.

According to the pn-junction compound semiconductor light-emittingdevice of the present invention, the conductor layer is composed of aboron-containing Group III-V compound semiconductor. Thus, in thepn-junction compound semiconductor light-emitting device, the conductorlayer and the light-permeable substrate are joined to each other withhigh adhesion. On the conductor layer, an Ohmic electrode can bereliably formed.

Therefore, the invention provides a pn-junction compound semiconductorlight-emitting device which has low resistance, allows device operationcurrent to easily flow, and exhibits excellent efficiency of extractionof light to the outside.

Since the conductor layer has a bandgap at room temperature which isgreater than that of the light-emitting layer, the light emitted fromthe light-emitting layer can be caused to transmit to thelight-permeable substrate with low transmission loss, whereby highemission intensity can be attained.

Since the conductor layer is composed of an undoped Group III-V compoundsemiconductor containing boron to which an impurity element has not beenintentionally added, the phenomenon that the added impurity elementdiffuses into the light-emitting layer or other layers, thereby varyingforward voltage or other properties of a pn-junction compoundsemiconductor light-emitting device does not occur, and low forwardcurrent can be attained.

Since the conductor layer is composed of a Group III-V compoundsemiconductor containing arsenic and boron, an electrode exhibitingexcellent Ohmic contact characteristics can be formed on the conductorlayer, whereby low forward current can be attained.

Since the conductor layer is composed of a Group III-V compoundsemiconductor containing phosphorus and boron or a Group III-V compoundsemiconductor containing arsenic and boron (boron arsenide phosphide), awide bandgap is obtained and the light emitted from the light-emittinglayer can be caused to transmit to the light-permeable substrate withlower transmission loss, whereby higher emission intensity can beattained.

Since the conductor layer is composed of a boron-containing Group III-Vcompound semiconductor containing twins, lattice mismatch between theconductor layer and a base layer is mitigated, thereby yielding aconductor layer with high crystallinity. Thus, a pn-junction compoundsemiconductor light-emitting device which has lower resistance andexhibits excellent efficiency of extraction of light to the outside canbe produced.

According to the method for producing a pn-junction compoundsemiconductor light-emitting device, an n-type or a p-type conductorlayer composed of a boron-containing Group III-V compound semiconductoris formed to provide a joining layer for joining the stacked structureand the light-permeable substrate. Thus, the conductor layer and thelight-permeable substrate can be joined to each other at low temperaturewith high adhesion without using a light-irradiation means such as YAGlaser for heating in combination.

According to the method, formation of a high-resistance layer composedof an oxide or another substance and thermal diffusion of an impurityelement added to a component layer of the stacked structure can beprevented, such phenomena conventionally occurring in the case where aconductor layer composed of gallium phosphide or a similar substance anda light-permeable substrate are joined at high temperature. On theconductor layer, an Ohmic electrode can be reliably formed.

According to the method, a pn-junction compound semiconductorlight-emitting device which has low resistance, allows device operationcurrent to easily flow, and exhibits excellent efficiency of extractionof light to the outside can be produced.

Since the crystal substrate is removed after joining of the conductorlayer to the light-permeable substrate, light absorption by the crystalsubstrate can be avoided, and a pn-junction compound semiconductorlight-emitting device which exhibits excellent efficiency of extractionof light to the outside can be produced.

According to the method, the conductor layer is formed through crystalgrowth at a growth rate of 20 nm/min to 30 nm/min until the conductorlayer thickness reaches 10 nm to 25 nm, followed by crystal growth at agrowth rate less than 20 nm/min until the conductor layer comes to havea thickness of interest. Therefore, twins are incorporated into theconductor layer, whereby a conductor layer with high crystallinity canbe formed.

Since crystal growth is continued at a growth rate less than 20 nm/minuntil the conductor layer comes to have a thickness of interest, aconductor layer having high surface flatness can be formed, whereby theconductor layer and the light-permeable substrate can be joined to eachother with high adhesion.

INDUSTRIAL APPLICABILITY

The present invention provides a pn-junction compound semiconductorlight-emitting device having a light-emitting layer composed of ann-type or a p-type aluminum gallium indium phosphide for emitting lightof various wavelength and particularly, a high-luminance LED for use ina display element or an electronic apparatus such as an opticalcommunication apparatus.

1. A method for producing a pn-junction compound semiconductorlight-emitting device comprising the steps of: forming a stackedstructure through sequentially stacking on a crystal substrate a lowercladding layer, a light-emitting layer composed of n-type or p-typealuminum gallium indium phosphide, an upper cladding layer, and ann-type or a p-type conductor layer composed of an undopedboron-containing Group III-V compound semiconductor, and a step ofjoining the conductor layer to a light-permeable substrate; wherein thecrystal substrate is removed after joining of the conductor layer to thelight-permeable substrate, and wherein the conductor layer is formedthough crystal growth at a growth rate of 20 nm/min to 30 nm/min andcontains twins, having, as a twining plate, a (111) lattice plane of theboron containing Group III-V compound semiconductor.
 2. The method forproducing a pn-junction compound semiconductor light-emitting deviceaccording to claim 1, wherein the conductor layer is formed throughcrystal growth at a growth rate of 20 nm/min to 30 nm/min until theconductor layer thickness reaches 10 nm to 25 nm, followed by crystalgrowth at a growth rate less than 20 nm/min until the conductor layercomes to have a thickness of interest.
 3. The method for producing apn-junction compound semiconductor light-emitting device according toclaim 1, wherein the conductor layer is composed of an undoped BP-basedsemiconductor.